Morphological counts of galaxies from early Hubble Space Telescope
imaging found a large increase in the number of
peculiar/irregular galaxies at fainter magnitudes (e.g.,
Driver et al. 1995;
Glazebrook et
al. 1995).
It was however unknown during
these early Hubble observations what the redshifts, and therefore the
characteristics, of these peculiar
galaxies were. When redshifts for these faint galaxies became available,
it was argued that Hubble types appeared in abundance by z ~ 1,
and evolve only slightly down to lower redshifts
(van den Bergh et
al. 2001;
Kajisawa & Yamada
2001).

Ultimately what is desired is a determination of Hubble types as a
function of redshift for galaxies of different luminosities and
stellar masses. This was performed for bright galaxies in the
Hubble Deep Field North and South by
Conselice et
al. (2004b).
The results of this are shown in
Figure 3
for galaxies brighter than I = 27.
As described in Section 3.1 there is a
rapid decline with increasing redshift in the number of normal
galaxies between z ~ 1 and z ~ 1.5, such that the co-moving
density increases by 8.3 × 103 Gpc-3
Gyr-1 for ellipticals and 5.7
× 103 Gpc-3 Gyr-1 for spirals during
this 1.6 Gyr period, roughly a factor of 10 increase in number densities.
As discussed briefly in Section 3.5 this
change in morphology is not caused by so-called morphological k-corrections
in which galaxies appear different at different wavelengths. It can however
be partially produced by selection effects, although a strong drop is
also found when considering galaxies at a fixed absolute magnitude
(Figure 2).
Both spirals and ellipticals with MB < - 20 should be found
in the Hubble Deep Fields up to z ~ 2, and at even higher redshifts
if passive evolution is considered (e.g.,
Conselice et
al. 2004b).

The redshift range 1 < z < 2 is obviously critical for
understanding
the final onset and production of the Hubble sequence and the origin of
the galaxy structure-redshift relationship. It is also the epoch (during a
short 2.5 Gyrs!) where the star formation rate, AGN activity and stellar
mass assembly is at its highest. Understanding how the galaxy
structure-redshift
relationship evolves during this epoch is critical for understanding
the causes behind
galaxy formation. It is therefore worth spending some time discussing
what is found morphologically and structurally in the galaxy
population between 1 < z < 2.

Figure 4 shows Advanced
Camera for Surveys (ACS) images of the brightest galaxies in the rest-frame
optical found within the Chandra Deep Field South Great Observatory Origins
Survey (GOODS) imaging. Clearly, there is a rich morphological mix
at this redshift, with many galaxies appearing similar to modern ellipticals
and spirals, but with important structural differences that make them
fundamentally different from modern normal galaxies. Another
way to investigate this population is to study systems that have spectral
energy distributions that likely place them at 1 < z < 2,
such as the extremely red objects, discussed in
Section 4.1. The images of these
galaxies reveal that some are almost normal, with outer shell like
features and what
appear to be large star forming complexes. Understanding the physical causes
behind these features will help reveal the formation mechanisms of galaxies.

Figure 4. The brightest galaxies in ACS
GOODS images whose photometric redshifts place them at 1 < z
< 2. These are ordered from brightest to faintest down to
MB = -21. The upper number is the MB of each
galaxy and the lower number is its redshift. There is a large
diversity of properties, from systems that appear very peculiar
to those that look similar to normal galaxies. Scale of these images is
~ 2" on each side, corresponding to ~ 17 kpc at these
redshifts. See Figure4.gif for a
high resolution version.